1. Field of the Invention
The present invention relates generally to electronic test equipment and more particularly to a testing apparatus for automatically determining the minimum discernable signal level of a pulse receiver under test.
2. Description of the Prior Art
It is, of course, obvious that an important measure of the performance of a pulse-type RF receiver is its ability to detect low level signals. Although the need to measure such performance is recognized, and even required on a daily basis for radar receivers under FAA control, it has been found that present measurement techniques are inadequate. This is due in part to the fact that such prior art measurement techniques are poorly or inconsistently defined and further because such tests are highly subjective.
One such technique for measuring the low signal level detection capabilities of a pulse-type receiver is the tangential sensitivity measurement technique. Typically, an oscilloscope is connected to the video output of a receiver under test and adjusted such that the noise generated by the receiver may be observed. Next, an appropriate source of pulse modulated RF energy of known amplitude is coupled to the input of the receiver and the resultant video pulse monitored on the oscilloscope. Finally, the level of the energy source is adjusted so as to align an imaginary line drawn through the average level of the negative noise peaks on the video pulse with an imaginary line drawn through the average level of the positive peaks of the baseline adjacent to the video pulse. The receiver sensitivity is then determined by reference to the power level of the reference energy source.
As indicated, this test is highly subjective in that it is dependent upon the arbitrary levels at which the imaginary lines are drawn. Further, the test is influenced by the brightness setting of the oscilloscope. This is because the excursions of high frequency noise signals are more apparent as this setting is increased.
Another test of receiver sensitivity commonly utilized is the minimum discernable signal or MDS test. Typically, an oscilloscope and an energy source configured as previously described are employed. For this test, the amplitude of the reference source is adjusted to a level at which the resultant video pulse is just discernable. Here, again, the test is highly subjective, being dependent upon how hard one tries to discern the pulse. By knowing where the pulse is or should be, and by watching the apparent behavior of the noise in this region long enough, one can become convinced that a low level signal which is not otherwise obviously apparent is indeed visible.
Tests conducted to determine the subjectability and repeatability of these tests have disclosed variations in excess of 3 dB. In other words, a particular radar receiver might both be determined to be acceptable and also be 3 dB below that level at which maintenance is required.
It is useful at this point to examine what information the tests are designed to provide. Most commonly, these tests are designed to provide some indication of what receiver driving signal amplitude is required to insure a given degree of probability of signal detection given an acceptable level of false alarms. More specifically, the false-alarm probability has been defined as that period of time, during nonsignal conditions, in which the noise developed at the output of the receiver exceeds a given threshold level divided by the total observation time. In similar fashion, the probability of detection has been defined as that period of time in which a signal developed by the receiver in response to a predetermined receiver driving signal exceeds that, or some other, threshold level divided by that period in which the responsive signal failed to exceed the threshold level.
In summary, indirect methods of determining the probability of detection of a receiver are used, which methods are both poorly defined and are highly subjective.